Computational Chemistry of Clusters and Crystals

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

So Hirata of the University of Illinois at Urbana-Champaign is supported by an award from the Chemical Theory, Models and Computational Methods program in the Chemistry Division, the Condensed Matter and Materials Theory program in the Division of Material Research and the Computational and Data-enabled Science and Engineering Program (CDS&E) to develop computational approaches and software for the study of molecular crystals. Molecular crystals are a large, important class of solids that consist of well-defined molecular units bound by weak interactions. They include nature's most abundant and important solids such as the ices of the atmospheric species of Earth and other planets. Synthetic chemists can fashion molecules that aggregate into superstructures, which, if crystalline, are also molecular crystals. Some explosives and many drugs fall into this category. Some molecular crystals display optical and electronic properties making them suitable for optoelectronic devices such as solar cells. The goal of this project is to develop a general computational method for molecular crystals and related ionic crystals as well as organic molecular superconductors. The principal investigator and his coworkers develop software to predict the structure, optical and thermal properties, and phase behavior of organic crystalline solids with unprecedented accuracy and applications in high-pressure chemistry, geochemistry, planetary science, and materials science. This research activity involves innovative education in physical chemistry. A series of physical chemistry lectures is recorded and made available online with a matching set of problems, releasing all face-to-face classroom hours for problem solving, student's explanations of solutions, and discussions. The energy of a molecular crystal is approximated as a sum of the energies of its constituent fragments embedded in the self-consistently determined electrostatic environment of the crystal. The fragment energies are, in turn, evaluated by sophisticated molecular ab initio electronic structure methods. This allows an accurate calculation of a variety of properties of solids under finite temperature and pressure (structure, equation of state, infrared, Raman, inelastic neutron scattering spectra, heat capacity, enthalpy, Gibbs energy) at such high levels of fidelity as second- and higher-order perturbation theory or coupled-cluster theory. This project implements this method into robust and well-documented software that exploits the method's natural parallelism and makes it available for the broader scientific community. Furthermore, the project extends this method to energy bands and ionic crystals as well as organic molecular (super) conductors. The underlying idea that enables these calculations is the linear-combination-of-molecular-orbital (LCAO) crystal-orbital theory, a coarse-grained extension of the LCAO molecular-orbital concept, which has dominated computational quantum chemistry since its inception. By expanding the wave function of an organic molecular crystal, for instance, as a linear combination of its charge configurations, which, in turn, are treated by the aforementioned embedded-fragmentation scheme, this method describes charge transfer between constituent molecular units in these solids and thus charge density waves, spin density waves, and metallic as well as possibly superconducting states.